This application claims the benefit of Japanese Application No. 2002-035570, filed Feb. 13, 2002, in the Japanese Patent Office, the disclosure of which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to quinomethane compounds, and in particular, to novel quinomethane compounds that are useful as an electron transport substance in an electrophotographic photoreceptor (hereinafter merely referred to as a xe2x80x98photoreceptorxe2x80x99), an organic electroluminesence (EL) device or the like.
2. Description of the Related Art
In recent years, as electronic devices using an organic compound have been introduced, many organic electrophotographic photoreceptors that use an organic photoconductive material have been proposed and put into practical use, since organophotoconductive materials are non-polluting, low in cost, and the photoreceptor properties may be adjusted due to the degree of freedom of material selection.
The photosensitive layer of an organic electrophotographic photoreceptor predominantly comprises a layer in which an organic photoconductive material is dispersed in a resin, and many structures have been proposed, for example, a layered type structure in which a layer in which a charge generation material is dispersed in a resin (charge generation layer) and a layer in which a charge transport material is dispersed in a resin (charge transport layer) are laminated sequentially, and a single layer type structure comprising a single layer in which a charge generation material and a charge transport material are dispersed together in a resin.
Of the above, a functionally separated layered type photoreceptor in which a charge transport layer is laminated on top of a charge generation layer as the photosensitive layer has excellent photoreceptor properties and durability, and hence has been widely put into practical use. A hole transport material is generally used as the charge transport material in the charge transport layer provided in such a functionally separated layered type photoreceptor, and hence such a photoreceptor is used in an electrophotographic apparatus that operates with a negative charging process. However, the negative corona discharge used in the negative charging process is unstable compared with a positive corona discharge. Moreover, the amount of ozone generated is large, and hence, there have been problems of adverse effects on the photoreceptor and adverse effects on the usage environment. An organic electrophotographic photoreceptor that can be used with a positive charging process is effective for resolving these problems.
To make a photoreceptor having excellent durability as described above be suitable for a positive charging process and have high sensitivity, it is necessary to use a substance having an excellent electron transporting function. Many such substances and photoreceptors using these substances have been proposed. For example, in Japanese Patent Application Laid-open No. 1-206349, Japanese Patent Application Laid-open No. 4-360148, Journal of the Society of Electrophotography of Japan, Vol. 30, p266 to 273 (1991), Japanese Patent Application Laid-open No. 3-290666, Japanese Patent Application Laid-open No. 5-92936, Proceedings of the Pan-Pacific Imaging Conference/Japan Hardcopy ""98, Jul. 15 to 17, 1998, J A Hall, Tokyo, Japan, p207 to 210, Japanese Patent Application Laid-open No. 9-151157, Papers from Japan Hardcopy ""97, Jul. 9 to 11, 1997, J A Hall (Otemachi, Tokyo), p21 to 24, Japanese Patent Application Laid-open No. 5-279582, Japanese Patent Application Laid-open No. 7-179775, Papers from Japan Hardcopy ""92, Jul. 6 to 8, 1992, J A Hall (Otemachi, Tokyo), p173 to 176, Japanese Patent Application Laid-open No. 10-73937, Japanese Patent Application Laid-open No. 4-338760, Japanese Patent Application Laid-open No. 1-230054, Japanese Patent Application Laid-open No. 8-278643, Japanese Patent Application Laid-open No. 9-190002, Japanese Patent Application Laid-open No. 9-190003, and Japanese Patent Application Laid-open No. 2001-222122, many electron transport substances and electrophotographic photoreceptors using such electron transport substances are proposed and disclosed, and have received attention. Moreover, in the case of a single layer type photosensitive layer, photoreceptors that use a combination of an electron transport substance and a hole transport substance as disclosed, for example, in Japanese Patent Application Laid-open No. 5-150481, Japanese Patent Application Laid-open No. 6-130688, Japanese Patent Application Laid-open No. 9-281728, Japanese Patent Application Laid-open No. 9-281729, and Japanese Patent Application Laid-open No. 10-239874, have received attention as having high sensitivity, and have been put into practical use in some cases.
Moreover, to obtain photoreceptors having better properties, the present inventors have also proposed photoreceptors that contain a substance having an electron transporting function (disclosed, for example, in Japanese Patent Application Laid-open No. 2000-75520, Japanese Patent Application Laid-open No. 2000-199979, Japanese Patent Application Laid-open No. 2000-143607, and Japanese Patent Application Laid-open No. 2001-142239).
Moreover, recently there are organic EL devices implemented as luminescent devices that use an organic photoconductive material and whose application to displays and the like is anticipated. With regard to these organic EL devices as well, many proposals have been made relating to improving the organic material, and practical application has been carried out in some cases.
The simplest structure of an organic EL device is a structure in which a luminescent layer containing a luminescent material that is an organic compound is sandwiched between electrodes; and by passing a current between the electrodes, electrons and holes are injected into the luminescent layer from the electrodes. Hence, excitons are formed in the luminescent layer, and then light is emitted when recombination occurs. Moreover, to inject electrons and holes efficiently into the luminescent layer from the electrodes, for example, a structure has also been proposed in which functional layers, namely a hole transport layer, a hole injection layer, an electron transport layer and an electron injection layer, are laminated together with the luminescent layer. Of these functional layers, an organic compound having an electron transporting function is used in the electron transport layer and the electron injection layer (see for example, Oyo Buturi, Vol. 70, No. 12 (2001), p1419 to 1425, xe2x80x98Development Trends for High-Efficiency Organic EL Materialsxe2x80x99 (Omori)).
However, with diphenoquinone compounds and stilbenequinone compounds that are already known as substances having an electron transporting function when used in an electrophotographic photoreceptor, electrical properties such as residual potential and sensitivity are not sufficiently satisfactory. Hence, electron transport substances having better electrical properties have been hoped for. Moreover, in organic EL applications as well, there have been calls for high-performance electron transport materials that have higher brightness than conventionally, and moreover enable the luminous efficiency to be improved.
It is thus an aspect of the present invention to provide compounds that have an excellent electron transporting function and are thus useful in electrophotographic photoreceptor or organic EL applications.
Additional aspects and advantages of the invention will be set forth in part in the description which follows and, in part, will be obvious from the description, or may be learned by practice of the invention.
To attain aspects of the invention, a compound comprises a quinomethane compound having a structure represented by the below-mentioned general formula (1): 
wherein R1 to R4 and R9 to R12 are the same or different and each represents a hydrogen atom, an optionally substituted alkyl group having 1 to 12 carbon atoms, an optionally substituted cyclic alkyl group, an optionally substituted aryl group, or an optionally substituted alkoxy group having 1 to 6 carbon atoms; R5 and R6 are the same or different and each represents an optionally substituted alkyl group having 1 to 12 carbon atoms, an optionally substituted aryl group, or an optionally substituted heterocyclic group; R7 and R8 each represents a hydrogen atom; and each of the optional substituents is a halogen atom, an alkyl group having 1 to 6 carbon atoms, an alkoxy group having 1 to 6 carbon atoms, an aryl group, or an optionally substituted heterocyclic group.